Behind the Forecast: Atmospheric traffic jams

Science Behind the Forecast: Atmospheric Blocks

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LOUISVILLE, KY (WAVE) - Atmospheric patterns typically repeat themselves but in the case of an atmospheric block, the same pattern may repeat for weeks. This could result in droughts, heat waves, droughts and various other weather extremes.

Meteorologists typically look for blocks on upper air charts - mainly in the mid to upper level of the atmosphere.

Think of atmospheric blocks as traffic jams. Blocks are most common with high-pressure, this is because areas of high pressure typically move slower than low-pressure systems and cover a large area. In some cases, a low-pressure system can also cause an atmospheric block.

There are five types of atmospheric blocks: Omega, Rex, Ring of Fire, Split-Flow, and Cut-off Low.

The Omega Block looks just like the Greek letter Omega (Ω). For the United States, this happens when a high-pressure ridge positions itself over the center of the United States with troughs, think low pressure, on both coasts. It's position forces air from the from Southwestern U.S. north into Canada and back south into the Southeast. Areas beneath the high have dry weather and light winds while those under troughs deal with cloudy conditions and rain. The left side of the block (west coast) has above normal temperatures while the right side (east coast) has below normal temperatures.

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A Ring of Fire occurs when an area of high pressure sits over the Southeastern U.S.  It can linger for days especially if the jet stream is weak and there aren't many cold/cool fronts. The "Ring of Fire" terminology comes from the location of thunderstorms in relation to the high pressure. Air is most stable at the high-pressure's center. At its edges, the atmosphere is unstable enough for afternoon thunderstorms to pop-up. This creates a ring of thunderstorms (fire) around the area of high pressure.

A Cut-Off Low can form anywhere in the country. They typically occur when the upper winds shift to a high latitude leaving an area of low pressure behind. They can bring persistent rain wherever they set up shop.

A Rex Block occurs when a high-pressure ridge sets up next to a strong low-pressure trough. The high is situated to the north of the low. It looks like a backward S or a half figure 8. Their positions prevent other weather systems from moving through. Air flows clockwise around the high then turns south to flow counterclockwise around the low. Since the air basically flows from north to south, the system does not move east quickly. They are most often found over western Europe and the U.S.'s west coast.

Split-Flows happen when the jet stream is split into two separate branches around an area of high-pressure. Weather systems can quickly flow through the jets but weaken as they do; the weather remains stagnant in between the two jets.

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